The initiation of an immune response against a specific antigen in mammals is brought about by the presentation of that antigen to T-cells. An antigen is presented to T-cells in the context of a major histocompatibility (MHC) complex. MHC complexes are located on the surface of antigen presenting cells (APCs); the 3-dimensional structure of MHCs includes a groove or cleft into which the presented antigen fits. When an appropriate receptor on a T-cell interacts with the MHC/antigen complex on an APC in the presence of necessary co-stimulatory signals, the T-cell is stimulated, triggering various aspects of the well characterized cascade of immune system activation events, including induction of cytotoxic T-cell function, induction of B-cell function and stimulation of cytokine production.
There are two basic classes of MHC molecules in mammals, MHC class I and MHC II. Both classes are large protein complexes formed by association of two separate proteins. Each class includes trans-membrane domains that anchor the complex into the cell membrane. MHC class I molecules are formed from two non-covalently associated proteins, the .alpha. chain and .beta.2-microglobulin. The .alpha. chain comprises three distinct domains, a .alpha.1, .alpha.2 and .alpha.3. The three dimensional structure of the .alpha.1 and .alpha.2 domains forms the groove into which antigens fit for presentation to T-cells. The .alpha.3 domain is a trans-membrane Ig-fold like domain that anchors the ax chain into the cell membrane of the APC. MHC class I complexes, when associated with antigen (and in the presence of appropriate co-stimulatory signals) stimulate CD8 cytotoxic T-cells, which function to kill any cell which they specifically recognize.
The two proteins which associate non-covalently to form MHC class II molecules are termed the .alpha. and .beta. chains. The .alpha. chain comprises .alpha.1 and .alpha.2 domains, and the .beta. chain comprises .beta.1 and .beta.2 domains. The cleft into which the antigen fits is formed by the interaction of the .alpha.1 and .beta.2 domains. The .alpha.2 and .alpha.2 domains are trans-membrane Ig-fold like domains that anchors the .alpha. and .beta. chains into the cell membrane of the APC. MHC class II complexes, when associated with antigen (and in the presence of appropriate co-stimulatory signals) stimulate CD4 T-cells. The primary functions of CD4 T-cells are to initiate the inflammatory response and to regulate other cells in the immune system.
The genes encoding the various proteins that constitute the MHC complexes have been extensively studied in humans and other mammals. In humans, MHC molecules (with the exception of class I .beta.2-microglobulin) are encoded in the HLA region, which is located on chromosome 6 and constitutes over 100 genes. There are 3 class I MHC .alpha. protein loci, termed HLA-A, -B and -C. There are also 3 pairs of class II MHC .alpha. and .beta. chain loci, termed HLA-DR(A and B), HLA-DP(A and B), and HLA-DQ(A and B). In rats, the class I .alpha. gene is termed RT1.A, while the class II genes are termed RT1.B.alpha. and RT1.B.beta.. More detailed background information on the structure, function and genetics of MHC complexes can be found in Immunobiology: The Immune System in Health and Disease by Janeway and Travers, Cuurent Biology Ltd./Garland Publishing, Inc. (1997) (ISBN 0-8153-2818-4), and in Bodmer et al. (1994) "Nomenclature for factors of the HLA system" Tissue Antigens vol. 44, pages 1-18 (with periodic updates).
The key role that MHC complexes play in triggering immune recognition has led to the development of methods by which these complexes are used to modulate the immune response. For example, activated T-cells which recognize "self" antigens (autoantigens) are known to play a key role in autoimmune diseases (such as rheumatoid arthritis and multiple sclerosis). Building on the observation that isolated MHC class II molecules (loaded with the appropriate antigen) can substitute for APCs carrying the MHC class II complex and can bind to antigen-specific T-cells, a number of researchers have proposed that isolated MHC/antigen complexes may be used to treat autoimmune disorders. Thus U.S. Pat. Nos. 5,194,425 (Sharma et al.) and 5,284,935 (Clark et al.) disclose the use of isolated MHC class II complexes loaded with a specified autoantigen and conjugated to a toxin to eliminate T-cells that are specifically immunoreactive with autoantigens. In another context, it has been shown that the interaction of isolated MHC II/antigen complexes with T-cells, in the absence of co-stimulatory factors, induces a state of non-responsiveness known as anergy. (Quill et al., J. Immunol., 138:3704-3712 (1987)). Following this observation, Sharma et al. (U.S. Pat. Nos. 5,468,481 and 5,130,297) and Clarke et al. (U.S. Pat. No. 5,260,422) have suggested that such isolated MHC II/antigen complexes may be administered therapeutically to anergize T-cell lines which specifically respond to particular autoantigenic peptides.
Methods for using isolated MHC complexes in the detection, quantification and purification of T-cells which recognize particular antigens have been studied for use in diagnostic and therapeutic applications. By way of example, early detection of T-cells specific for a particular autoantigen would facilitate the early selection of appropriate treatment regimes. The ability to purify antigen-specific T-cells would also be of great value in adoptive immunotherapy. Adoptive immunotherapy involves the removal of T-cells from a cancer patient, expansion of the T-cells in vitro and then reintroduction of the cells to the patient (see U.S. Pat. No. 4,690,915; Rosenberg et al. New Engl. J. Med. 319:1676-1680 (1988)). Isolation and expansion of cancer specific T-cells with inflammatory properties would increase the specificity and effectiveness of such an approach.
To date, however, attempts to detect, quantify or purify antigen specific T-cells using isolated MHC/antigen complexes have not met with widespread success because, among other reasons, binding between the T-cells and such isolated complexes is transient and hence the T-cell/MHC/antigen complex is unstable. In an attempt to address these problems, Altman et al. (Science 274, 94-96 (1996) and U.S. Pat. No. 5,635,363) have proposed the use of large, covalently linked multimeric structures of MHC/antigen complexes to stabilize this interaction by simultaneously binding to multiple T-cell receptors on a target T-cell.
Although the concept of using isolated MHC/antigen complexes in therapeutic and diagnostic applications holds great promise, a major drawback to the various methods reported to date is that the complexes are large and consequently difficult to produce and to work with. While the complexes can be isolated from lymphocytes by detergent extraction, such procedures are inefficient and yield only small amounts of protein. The cloning of the genes encoding the various MHC complex subunits has facilitated the production of large quantities of the individual subunits through expression in prokaryotic cells, but the assembly of the individual subunits into MHC complexes having the appropriate conformational structure has proven difficult.